The invention relates to a bi-directional signal transmission system, for transmission of a binary signal in both directions. The invention further relates to an interface for such a system.
An example of a bi-directional signal transmission system is the well-known 12C-bus system, disclosed in, for instance, Philips Data Handbook IC12a, "12C-bus compatible ICs, Types MAB84XI family to PCF8579", 1989, pp. 31-53 and "The 12C bus and how to use it (including specifications)" 1995 by Philips Semiconductors. The 12C-bus system is a serial bus system with stations, that is, integrated circuits, that are interconnected through a bi-directional two-wire transmission channel, one wire for a binary data signal, and another for a binary clock signal, and that communicate according to a predetermined protocol. Dependent on its particular function within the system, each station can operate as transmitter or receiver, or both.
The data wire as well as the clock wire are connected to a positive supply voltage through pull-up resistors. The wires are floating high, when the stations do not communicate. Each station's output stage has an interface to the bus with an open collector or an open-drain which enables any one of the stations to pull down the voltage level on the bus to low.
The capacitive load of the 12C bus determines the length of the bus and the number of stations that can be connected to the bus. The bus capacitance is the total capacitance of wire, connections and IC pins. If the bus capacitance is increased, the bus will have to be run at lower speeds. Moreover the rise and falls of the bus become a problem.
It has been proposed that a bi-directional signal transmission system having a bus interface device may alleviate these problems. In these proposals the bus interface device is coupled to a first bi-directional data bus on one side and coupled on the other side to two logic paths, one for receiving and one for transmitting signals. These two logic paths being coupled to a second bi-directional data bus. However the systems so far proposed have major limitations and very restricted application. Some suffer from the problem of latching, namely when the receiving logic path goes low, the transmitting logic path on the bus interface goes low due to the bi-directional nature of the first data bus and the transmitting logic path just stays low. The bus is `latched` in this low state. Others are conditionally stable but produce spurious logic signals and are prone to oscillate.